This invention relates to an improved process for the production of particulate coal-in-liquid mixtures and to the products produced by the process.
The ever-increasing cost and uncertainty of supply of oil on the world market has generated renewed interest in coal as a fuel source. Recent developments in coal processing technology have included new and improved methods of coal gasification and hydrogenation to produce liquid fuels and chemicals. Also, efforts are being made to demonstrate the feasability of replacing oil with coal-oil mixtures for use in both electric power generation and in industrial heating and steam generation. These applications of coal involve mixing the coal in a finely divided state with a variety of liquids. For example, certain gasification processes call for the mixing of comminuted coal with various grades or types of oil; hydrogenation requires mixing finely divided coal with a hydrogen donor solvent; and the preparation of coal-oil mixtures, as the name implies, involves the mixing of particulate coal with oil. Also, coal is often combined with water to produce a pumpable slurry for transportation from the mine to coal processing units or power plants.
A major problem attendant the use of coal-in-liquid mixtures is that the coal has a tendency to settle out of the liquid during transportation or storage. This problem most often manifests itself in the plugging of feed lines of the coal processing equipment or in the fouling of pumps and valves in storage vessels and transportation pipe lines. Various solutions to the settling out problem have been proposed, such as the addition of stabilizing agents to the coal-in-liquid mixtures, the employment of mechanical stirring devices in storage tanks, the careful regulation of flow rates during transportation, and the employment of processing equipment designs which minimize the problem, such as laying out of piping to avoid dead spots, vertical mounting of valves, and the like. However, all of these proposed solutions translate into increased construction or operating costs for the particular process involved. In addition, the use of stabilizing agents tends to diminish the reactivity of the coal particles, making subsequent processing thereof less efficient.
Another problem associated with the use of particulate coal-in-liquid mixtures is that the particulate component of the mixtures, which includes ash particles generated during milling of the coal, cause rather severe erosion of the pumps, valve, pipes, feedlines, and burners of coal processing or combustion apparatus. The ash particles, being harder than the coal particles, are particularly abrasive. Erosion becomes especially pronounced when the particle sizes are on the order of 40 microns or greater, which is typically the mid-size of coal pulverized in air-swept mechanical grinding mills.
Coal-in-liquid mixtures containing coal particles of greater than 40 microns have other disadvantages, as well. For example, in hydrogenation processes, the larger the coal particles, the longer they take to liquify, which results in long reactor residence times and an excessive amount of unreacted coal in output streams. Similarly, when these relatively large size particles are present in composite fuels, such as coal-oil mixtures, they do not undergo complete combustion in the normal furnace residence periods available in conventional oil and gas furnaces. As a result, the ash particles impinge on surfaces of the furnace, e.g., boiler tubes, in a slagging condition.
The development of methods for producing stable particulate coal-in-liquid mixtures having uniformly small particle sizes, as well as reduced ash particle content, continues to be a highly desired objective.